1. Field of the Invention
The present invention relates to a method of forming a thin-film resistor, and more particularly, to a method of forming a thin-film resistor in a semiconductor wafer.
2. Description of the Prior Art
In semiconductor integrated circuit design, a simple resistor is often made from a gate conductive layer or an impurity-doped layer in a predetermined area of the semiconductor wafer. However, the resistance typically obtained from the gate conductive layer and the impurity-doped layer is often too low. To obtain a sufficient level of resistance, the surface area of the resistor must be greatly increased. Also, the resistance of the silicon-containing gate conductive layer and impurity doped layer varies with temperature changes which results in instability of the resistance value. Therefore, a stable thin-film resistor with lower conductivity on the semiconductor wafer is needed to meet design requirements.
Please refer to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 are schematic diagrams of a method of forming a thin-film resistor according to the prior art. A thin-film resistor 20 is formed on a dielectric layer 12 of a semiconductor wafer 10. As shown in FIG. 1, a thin-film resistor 20 is produced by first sequentially forming a resistance layer 14 and a protective layer 16 in a predetermined area on the surface of the dielectric layer 12. Then, a conductive layer 18 made of an aluminum alloy is formed on the protective layer 16. Next, a large portion of the conductive layer 18 and protective layer 16 positioned on the resistance layer 14 is removed by wet-etching with the remaining portion at the two ends of the resistance layer 14 functioning as two electrical terminals of the two ends of the resistance layer 14. This is illustrated in FIG. 2.
The wet-etching process is an isotropic etching process with equal horizontal and vertical etching depths. To define the conductive layer 18 properly through wet etching, the surface area of the resistance layer 14 and the protective layer 16 must be large. Only if the resistance layer 14 and protective layer 16 is large can a large portion of the conductive layer 18 and protective layer 16 be removed while still preserving the two portions at the ends of the resistance layer 14. Although this method can be utilized in processing gate widths greater than 3 micrometers, it is ineffective in processing narrower gate widths.
It is therefore a primary objective of the present invention to provide a method of forming a thin-film resistor in a semiconductor wafer with much more stable resistance. Also, the method is utilized in gate processing for forming narrower gates.
In a preferred embodiment, the present invention provides a method of forming a thin-film resistor on a dielectric layer of a semiconductor wafer, the method comprising:
forming a resistance layer, a buffering layer and a protective layer in a predetermined area of the dielectric layer, the buffering layer being positioned on the resistance layer and below the protective layer for buffering the effect of thermal stress exerted on the resistance layer by the protective layer;
forming an insulating layer on the semiconductor wafer to cover the upper and side surface of the protective layer, the side surface of the buffering layer and the resistance layer, and the surface of the dielectric layer outside the predetermined area;
performing a dry-etching process on the insulating layer to form two openings extending down to the protective layer over which the protective layer is used for protecting the resistance layer from plasma damage caused by the dry-etching process;
performing a first wet-etching process on the protective layer below the two openings of the insulating layer to form two openings extending down to the buffering layer;
performing a second wet-etching process on the buffering layer below the two openings of the protective layer to form two openings extending down to the resistance layer; and
forming two conductive layers in the two openings of the insulating layer, the protective layer and the buffering layer to contact the two ends of the resistance layer wherein the two conductive layers are used as two electrical terminals for connecting the two ends of resistance layer.
It is an advantage of the present invention that the thin-film resistor thus formed comprises a resistance layer below a buffering layer and a protective layer. The buffering layer buffers the thermal stress exerted on the resistance layer and the protective layer protects the resistance layer from plasma damage. The resulting thin-film resistor has a much more stable resistance.
This and other objective of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various figures and drawings.